One of the most difficult problems in the preparation of coal is reducing the sulfur content of the coal, which normally can exist in three forms: organic; pyritic; and as sulfate. Since the organic sulfur is distributed through the coal matrix as an integral part of the coal molecular structure, its removal requires chemical treatment. Pyritic sulfur may be removed by physical cleaning processes to various extents. The sulfate sulfur, except for oxidized or weathered coals, is usually less than 0.1% and is not usually an important factor in the clean-up of coal.
Thus, there are two types of desulfurization processes for coal prior to its utilization: chemical processes directed usually to oxidation or reduction of sulfur; and physical processes. The chemical processes include those which are conducted at elevated temperatures using gases such as hydrogen, nitrogen, chlorine, steam, air, etc., or with solutions such as sodium hydroxide, ferric sulfate, cupric sulfate, etc. These chemical methods are capable of removing both the organic and inorganic sulfur, but the degree of desulfurization depends upon the particular method. A disadvantage of chemical processes is that they usually cause decrease in coal volatile matter and in the heating value of the coal. Furthermore, the operating costs of chemical methods can be prohibitively expensive and the processes themselves can, in turn, produce toxic waste products.
It is therefore important to develop advanced physical methods of cleaning coal, such as by improving the conventional froth flotation methods, selective flocculation methods, oil agglomeration methods, magnetic separation methods, etc. One of the drawbacks is that physical cleaning methods usually remove only the part of the pyritic sulfur which can be liberated during coal comminution. The degree of mineral matter (pyrite) removal is dependent upon the mineral matter size, its distribution, the pyrite particle size, and other physical characteristics of the raw coal.
Oil agglomeration, one of the conventional physical cleaning methods, is based on the principle that coal particles are hydrophobic, or at least less hydrophilic, than the inorganic materials in the coal and therefore can be agglomerated and separated from mineral matter by addition of a suitable bridging liquid which wets the carbonaceous (hydrophobic) constituents.
However, studies show that poor pyrite rejection in oil agglomeration of coal is observed. See Gregory, I. W., "Oil Agglomeration of Coal Fines", Mining Technology Clearinghouse, R & D Commentary, 1982; Mezey, E. J., "Application of Oil Agglomeration to Coal Wastes", United States EPA Report. EPA 600/7-79-025C, 1979. In an attempt to overcome this problem, a number of methods for altering the surface characteristics of pyrite have been investigated, which are based on the alteration of the pyrite surface to render it hydrophilic.
Perrott, et al., Chemical and Metallurgical Engineering, 25 (5), 182-188 (1921), disclose the wet grinding of powdered coal, water and oil wherein the oil comprises 30% by weight of coal, and agitation of a slurry of these components to form an amalgam of oil and coal which can be separated from the water, ash and other hydrophilic components. In another process discussed by Perrott, et al. in the same publication, a slurry of powdered coal, water and oil wherein the oil comprises 25% by weight of coal is agitated and the coal/oil amalgam is separated from the hydrophilic components and ash by screening on a 100 mesh screen. By this process the pyritic sulfur reduction in bituminous coal is exemplified as being reduced from 3.01% in a bituminous feed coal to 2.10% in the recovered amalgam. In both of the aforementioned processes, the amount of oil is greater than 10% by weight of the coal and the reported pyritic sulfur reduction is minimal.
Canadian Patent No. 1,144,500 discloses the agitation of a slurry of oil, coal and water containing up to 50% by weight solids to form coal agglomerates. The agglomerates are separated by screening and the volatiles from the agglomerates are extracted by flashing. The agglomerates are merely separated from the hydrophilics and inorganics by screening and there is no disclosure of an agitation-aeration-separation wash cycle as disclosed herein which advantageously removes pyrite.
U.S. Pat. No. 3,856,668 discloses the screening of coal particulates in a water slurry containing about 2-10% hydrocarbon by weight of coal (preferably 3-7% if the hydrocarbon is a heavy oil). There is no disclosure of an agitation-aeration-separation wash cycle which advantageously removes pyrite according to the present invention.
Additional difficulties regarding pyrite rejection during oil agglomeration are the problem of dealing with the different degrees of dissemination of pyrite in the coal matrix, depending on the type and source of the coal; and the presence of other minerals, such as chalcopyrite (CuFeS.sub.2) in the pyrite. Usually a high degree of dissemination of the pyrite in the coal makes the pyrite removal more difficult and also the incorporation of chalcopyrite into the pyrite may change the surface properties of the pyrite in terms of its response to oxidation or action of depressing agents. It has therefore been found that successful modification of the oil agglomeration technique for pyrite rejection in a specific coal might be totally unsuitable for a different coal.
It is an object of the present invention to provide a method for causing pyrite rejection from bituminous coals.
It is another object of the present invention to provide a method which substantially reduces the ash content of bituminous coals.
These and other objects of the present invention will be apparent from the following description of the preferred embodiment and the appended claims and from practice of the invention.